1. Field of the Invention
The present invention relates to a method for direct heat treating a steel wire rod, more particularly, to a method for direct heat treating a hot rolled wire rod in direct sequence with the final finishing stand of a hot rolling mill by utilizing the retained heat of the hot rolled rod.
2. Description of the Prior Art
The conventional high carbon steel wire rods produced by the hot rolling mill has been subjected to air-patenting (referred to AP hereinafter) or lead-patenting (referred to LP hereinafter) prior to the wire drawing step with a view to enhancing its drawability, tensile strength and toughness.
Recently the direct heat treatment of the hot rolled rod wherein the retained heat of the rod as it emerges from the final finishing stand of the hot rolling mill is fully utilized (referred to DP hereinafter) has been carried out. As is well known, the LP wire rod has a high strength as well as high toughness, its metallurgical structure is completely transformed to sorbite, and further it is considerably superior in uniformity along the whole length thereof. In accordance with miscellaneous method of DP of prior art heretofore carried out or proposed, the rod quality as equal as that resulting from AP can be attained. However, the rod quality equal to that of LP has never been achieved yet.
In order to obtain the strength and toughness of a rod equal to those of LP by the DP process, a cooling rate after hot rolling, particularly, cooling below Ar.sub.1 is most important, and a controlled cooling to a pertinent temperature should be effected at a pertinent cooling rate of 10.degree.-100.degree. C./sec. depending on the grade, type, and size of steel rod.
If the controlled cooling described above is effected immediately after the final finishing stand of the hot rolling mill or soon after the cooling guide pipe, however, it has been found that the rod quality equal to that obtained from LP is not achieved by the conventional DP process because the microstructure of the rod after the controlled cooling is not uniform due to the difference of temperature, which will be mentioned hereinafter and which lies in the directions of both section and length of the rod prior to the controlled cooling.
To eliminate the difference of structure of the rod after the controlled cooling, it is required that the temperature of the rod should be uniform along its length as the controlled cooling starts at a temperature above Ar.sub.1. Japan Patent Publication No. SHO45-23215(1970), Japan Patent Publication No. SHO46-19767(1971), and Japan Open-Laid Patent No. SHO53-149,811(1978) have all disclosed how to attempt the maintenance of the rod temperature along its temperature above Ar.sub.1 as one inventive idea, respectively. The Japan Patent Publication No. SHO45-23215(1970) makes an attempt to keep the rod temperature as uniform as possible by subjecting the hot rolled rod traveling on a reeler after hot rolling to the spray cooling of multiple stage type. However, the temperature difference of the rod before it is reeled is so large that it amounts as much as 10%, and it is difficult for the maintenance of uniform temperature, particularly where the high speed rolling line prevails in recent days.
Besides, although the uniform temperature of the rod can be attained before it is reeled, it is feared that the difference of temperature may occur at the start of controlled cooling after reeling. Japan Patent Publication No. SHO46-19767(1971) and Japan Open-Laid Patent No. SHO53-149,811(1978) disclose that both attempt to prompt the growth of austenite crystal grain by holding or heating the rod in the austenite zone, and it is required to heat the rod at a relatively higher temperature than Ar.sub.1, and particularly, at a temperature of above A.sub.3 +50.degree. C. in the disclosure of Japan Patent Publication No. SHO46-19767(1971), hence it causes excessive scale and also disadvantage in a descaling step, and further, it causes the degraded surface of the rod.